1. Field of the Invention
The present invention relates to an active matrix type display apparatus of transmission type using a thin film transistor (TFT) array.
2. Description of the Prior Art
Currently, a display apparatus using a liquid crystal material, an electroluminescent (EL) film or the like, which is a large-capacity and high-density active matrix type display apparatus used for television or graphic display, has been developed for practical use. In such a display apparatus, a semiconductor switch for driving and controlling each pixel is used so as to perform high-contrast display without cross-talk. MOSFETs formed on a single crystalline Si substrate or TFTs formed on a transparent substrate which can perform transmission type display are used as such semiconductor switches.
FIGS. 1 and 2 show an equivalent circuit of an active matrix type display apparatus having a TFT array and a schematic sectional view thereof, respectively. In FIG. 1, reference numeral 11 denotes a TFT as a switching element; and 12 denoter a display pixel electrode which is connected to the source of the TFT 11 through a connecting portion 10. The connecting portion 10 is a small region which connects the source of the TFT 11 with the display pixel electrode 12 having a large area. The connecting portion 10 can be integrally formed with the display pixel electrode 12 and the source electrode of the TFT 11, or can be formed of a different conductive material. Gates of the TFTs 11 of each row are commonly connected to respective gate buses Y1, Y2, . . . , Yn. Drains of the TFTs 11 of each column are commonly connected to respective drain buses X1, X2, . . . , Xm. Reference numeral 13 denotes an opposing electrode; and 14, a liquid crystal layer. In FIG. 2, a first electrode substrate 15 comprises a transparent insulating substrate 16 formed of, e.g., glass on which the TFT array (not shown) and the display pixel electrode 12 driven thereby are formed. A second electrode substrate 17 comprises a transparent insulating substrate 18 formed of, e.g., glass on which the opposing electrode 13 formed of a transparent conductive film is formed. The liquid crystal layer 14 is sandwiched between these first and second electrode substrates 15 and 17. Reference numeral 19 denotes a spacer or sealing portion.
A specific structure of one pixel portion is as shown in FIGS. 3 and 4. FIG. 3 is a plan view showing a pattern of the first electrode substrate 15, and FIG. 4 is a detailed sectional view thereof. The first electrode substrate 15 can be obtained in the following manner. A drain electrode 20 formed of a transparent conductive film, the display pixel electrode 12 and a source electrode 21 integrally formed with the connecting portion 10 are formed on the transparent insulation substrate 16. A semiconductor thin film 22 is deposited on the thus obtained structure. Then, a gate electrode 24 formed of, e.g., an Al film is formed on the semiconductor thin film 22 through a gate insulating film 23. Thereafter, a passivating film 25 formed of, e.g., an SiO.sub.2 film is formed on the resultant structure, thus obtaining the first electrode substrate 15.
In the above structure, the operation of the liquid crystal display apparatus is performed in the following manner. The gate buses Y1, Y2, . . . , Yn are sequentially scanned and driven by a scanning signal. The TFTs 11 are sequentially turned on by each gate bus during a time period of T.sub.f /n. Note that "T.sub.f " here means a frame scanning period. When, for example, m parallel image signal voltages are applied to the drain buses X1, X2, . . . Xm in synchronism with this scanning operation, the signal voltages are sequentially coupled to the display pixel electrodes 12 by each gate bus. Then, the signal voltage excites the liquid crystal layer 14 sandwiched between the display electrode 12 and the opposing electrode 13, thereby performing image display.
In this display apparatus, a TFT is easily subjected to light influence. Particularly, when a hydrogenated amorphous silicon film (a-Si:H) is used as a semiconductor film, conductivity becomes more than thousand times of that upon irradiation with light. When the TFT is turned off, therefore, a leakage current is increased, thereby degrading performance of a transmission type display apparatus. In order to compensate for this disadvantage, a light shielding layer must be provided on the TFT portion.
In addition, in this display apparatus, the signal voltage supplied through the TFT 11 and the connecting portion 10 is stored by a capacitor consisting of the display pixel electrode 12, the opposing electrode 13 and the liquid crystal layer 14. However, in many applications, since a leakage current flows in the liquid crystal layer 14, the stored signal voltage is attenuated before the TFT 11 is next driven. In order to prevent this disadvantage, a supplemental storage capacitor is connected in parallel to the capacitance of the liquid crystal layer 14. In this case, a capacitor electrode 26 formed of a transparent conductive film is formed on the transparent insulating substrate 16, as shown in FIG. 5. Then, an insulating film 27 formed of, e.g., an SiO.sub.2 film is formed on the capacitor electrode 26. Thereafter, the TFT array and the display pixel electrode 12 are formed in the same manner as in FIG. 4.
However, when a light shielding layer and a capacitor electrode are separately provided as described above, the number of manufacturing steps is increased, thereby degrading a yield of the display apparatus.